5 Things Every Human Should Know About Light

Rhett Allain

2015 is the International Year of Light (but the universal year of light is in 2533). I think we can all agree that light is very important in just about all aspects of our lives. But what are the key things that every human should know about it? Ok, I am going to give a pass on humans under the age of 7. Younger kids probably know lots of things about light (since they use it all the time) – but let’s just not worry about them for now. So, if you are over the age of 7, pay attention.

Light Is a Wave

What is a wave? Let’s start with an example. Toss a rock into a puddle and you will see something like this:

Slow motion video of waves in a puddle.

The rock makes a disturbance in the water (a ripple). This disturbance moves radially outward from where the rock hit. It is the disturbance moving outward, not the water molecules – they just move up and down. You can do something similar with a length of string. Place it on the floor and continue to shake one end. The disturbance travels down the string (a heavy string works better).

Let’s look at the case of a wave traveling down a string. There are 4 properties that you could consider:

These 4 properties are:

Amplitude: this is the size of the displacement for the disturbance. The units for amplitude depend on the type of wave. For a string, the units would be meters.

Wave speed: if you were to watch one displacement, it would be moving. The wave speed is the speed (that seems redundant). The unit for wave speed is meters per second.

Wavelength: this is the distance from one disturbance to the next measured in meters

Frequency: if you were to count how many waves passed a stationary point in each second, that would be the frequency (in cycles per second or Hertz).

The last three properties are related. The speed of the wave is equal to the product of the wavelength and the frequency.

So, light is a wave. This means that it has all the above properties and can do wavey things such as:

Expand and radiate in all directions (like a lightbulb or the waves in the water caused by a rock).

Interfere with other waves.

Bend around corners (yes, light does this – but it’s difficult to see).

Carry energy and momentum.

Interact with matter.

Light does all these things.

Light is an Electromagnetic Wave

You can make your own wave. Take a long extension cord and stretch it out on the ground. Now give one end a vertical shake. You should get something like this (this gif is in slow motion):

Now take away the extension cord and repeat the demonstration. Yes, nothing happens. If you don’t have a medium for a wave to travel in, there is no wave. But what about light? Light’s a wave, right? Yes indeed (as I described above). So, then how does light travel through empty space as it goes from the Sun to the Earth? What is the medium for a light wave?

It turns out there are two important things about electric and magnetic fields. First, here is a wire carrying electric current over a magnetic compass. The electric current makes a magnetic field which causes the compass needle to turn.

But you don’t even need an electric current to make magnetic fields. It turns out that a changing electric field will also make a magnetic field. Here is a coil of wire connected to a lightbulb (no battery). When it is placed over this changing magnetic field, the changing electric field is created that drives a current.

So we have a changing electric field that creates a magnetic field and a changing magnetic field creates an electric field. Put these two ideas together and you can make two waves (and electric field wave and a magnetic field wave) that both make the other propagate. Electromagnetic waves don’t need a medium because in a sense, they are their own medium.

Different Wavelengths of Light Interact Differently With Matter

First, there is the electromagnetic spectrum. You can make an electromagnetic wave of all different wavelengths – from larger than 1 meter (radio waves) to less than 10 picometers (gamma rays – but they are still waves). Here is the common classification of the electromagnetic spectrum going from large wavelengths to small.

Radio

Microwaves

Infrared

Visible light

Ultraviolet

X-rays (but they are waves)

Gamma rays

All of these are electromagnetic waves and they all travel at the same speed (the speed of light). However, they have different interactions with matter. If you are inside, your mobile phone can still get data from a cell tower since these radio waves pass through most walls. Can you see through the walls? No. Visible light does not pass through most walls. X-rays mostly go through your skin, but you can’t see (with visible light) through skin – that would just be weird.

Technically the interaction with light and matter depends on the frequency of light – but since frequency and wavelength are related, we can just talk about the wavelength.

You See Things When Light Enters Your Eye

Ok, this isn’t just about light but also how humans work.

There are two ways that light could enter your eye. First, there could be a light source (like a light bulb) that create light. This light then travels into your eye and BOOM – your brain interprets this signal as light. The other way (more common) is to see things by reflected light. Suppose you are looking at a pencil. The light (from somewhere) reflects off the pencil and then into your eye.

The human is smiling. You would smile too if you could see that pencil.

But what happens if there is no light that enters your eye? What if you are in a place with absolutely no source of light? In that case, you perceive the color black. Actually, this can be a fun question. Ask someone this:

Have you ever been somewhere with absolutely no light? (most people haven’t) If you are in a room that that is completely dark, what would you see? What happens after you wait a long, long time?

One of the very common answers is that you will see everything as black – at first. These humans will also say that after some time your eyes will adjust and then you WILL see something. The correct answer is that you will just see black – forever. If there is no light entering your eye then you just see black. The common idea is based on a common experience. Normally if you are in a dark room your eyes DO indeed adjust. However, this only works in rooms with a little bit of light – and there is almost always at least a little bit of light.

All Objects Produce Light

Maybe I should say that all objects create electromagnetic waves – they do. Let’s look at an example from your house. Go in your kitchen and turn the stove on high (assuming you have an electric stove). Now just watch it as it gets hotter (but don’t touch it).

Eventually, the stove element will get so hot it will glow a low reddish color (red hot). But actually, the element was producing light the whole time. It’s just that at lower temperatures the electromagnetic light is in a wavelength that you can’t see – it’s in the infrared spectrum.

Most things that you see around you emit EM (electromagnetic) radiation in the infrared spectrum – so you can’t see it. Well, you actually can indirectly see it if you have an awesome infrared camera (for your phone). These thermal cameras dectect the infrared light and create a false-color image that humans can see. For the most part, different colors in the IR image correspond to different temperatures of the objects.

Here is an example. This is my dog on a smooth floor. Notice that his eyes and nose are warmer than other parts of his body. Also notice that you can see his infrared reflection on the floor.

IR image of my dog using the Therm-App camera. Rhett Allain

But does this work for objects hotter than your stove element? Yes. As an object gets even hotter it creates light with shorter and shorter wavelengths. Eventually the object would look white as more of the shorter wavelength light is produced. Yes, it could even create ultraviolet light at even higher temperatures.